Method of annealing sheets of glass on a decreasing temperature gas support



July 25, 1967 J. c. FREDLEY ETAL 3, -q

METHOD OF ANNEALING SHEETS 0F GLASS ON A DECREASING TEMPERATURE GAS SUPPORT l7 Sheets-Sheet 1 Original Filed Nov. 9, 1962 mv TORS JAMES c. refiner M GEOKGE 4-. sum/r511 HT ORIVAY y 1967 J. c. FREDLEY ETAL 3,332,761

METHOD OF ANNEALING ETS OF GLASS ON A DECREASING SHE TEMPERATURE GAS SUPPORT Original Filed Nov. 9, 1962 17 Sheets-Sheet 2 .14 m on INVENTORS JAMES c. Fk'bl'Y 050205 a SLE/Gl/TEE y 25, 1967 J c. FREDLEY ETAL 3,332,761

METHOD OF ANNEALING SHEETS OF GLASS ON A DECREASING TEMPERATURE GAS SUPPORT Original Filed Nov. 9, 1962 17 sh t sh t 5 FIG.3

y 25, 1967 J. c. FREDLEY ETAL 33 METHOD OF ANNEALING SHEETS OF GLASS ON A DECREASING TEMPERATURE GAS SUPPORT Original Filed Nov. 9, 1962 17 Sheets-Sheet 4 NTS IN V E JAMES C. FKEDLEY 6501665 5. SLE/GHTE/Q BY ATTORNEY y 1967 J. c. FREDLEY ETAL 3,332,761

METHOD OF ANNEALING SHEETS OF GLASS ON A DECREASING TEMPERATURE GAS SUPPORT Original Filed Nov. 9, 1962 17 Sheets-Sheet 5 :Ijfili: 7 u JUB i Pi mmvrons JAMES c msazer BYGA-ORGE 4-. sum/ rm ArroeA/EY Ju y 25, 96 J. c. FREDLEY ETAL 3,332,761

METHOD OF ANNEALING SHEETS OF GLASS ON A DECREASING TEMPERATURE GAS SUPPORT l7 Sheets-Sheet 6 Original Filed Nov. 9, 1962 JAMES c FAEDLEY By aroma s. SL'EIGI/TER w UF ATTOR/Vf) FREDLEY ETAL .1. c. 3,332,761 METHOD OF ANNEALING SHEETS OF GLASS ON A DECREASING TEMPERATURE GAS SUPPORT July 25, 1967 17 Sheets-Sheet 7 Origi na1 Filed Nov. 9, 1962 HTTOENA'Y July 25, 1967 J. c. FREDLEY EQTAL. 3,332,761

METHOD OF ANNEALING SHEETS OF GLASS ON A DECREASING TEMPERATURE GAS SUPPORT Original Filed Nov. 9, 1962 17 Sheets-Sheet 8 INVENTO Jane: C. f'kifltf) smear 5. alt/Mme 3,332,761 METHOD OF ANNEALING SHEETS OF GLASS ON A DEGREASING J. C. FREDLEY ETAL July 25, 1967 TEMPERATURE GAS SUPPORT l7 Sheets-Sheet 10 Original Filed Nov. 9, 1962 3 UFu B lfllllllllllllll lllllllllfllll IIIIIII INVENTORS mew ET 050265 5 anal/r52 BY mid:

y 25, 1967 J. c. FREDLEY ETAL 3,332,761

METHOD OF ANNEALING SHEETS OF GLASS ON A DECREASING TEMPERATURE GAS SUPPORT l7 Sheets-Sheet 11 Original Filed Nov. 9, 1962 JAM: c r i k 050x55 2-. SLE/Gl/TEBQ BY ly 1957 J. c. FREDLEY ETAL METHOD OF ANNEALING SHEETS OF GLASS ON A DECREASING TEMPERATURE GAS SUPPORT l7 Sheets-Sheet 12 Original Filed Nov. 9, 1962 r 1 ago/20s a s2 BIG/{75R A Tram/4v INVENTO JAMES C. FRE'DLE y 25, 1967 J. c. FREDLEY ETAL 3,332,761

METHOD OF ANNEALING SHEETS OF GLASS ON A DEGREASING TEMPERATURE GAS SUPPORT Original Filed Nov. 9, 1962 17 Sheets-Sheet l5 j ANN-Om RADIANT HEATING GLASS MOVEMENT 5 UNIFORM PRESSURE UNIFORM GAS TEMPERATURE FOR CONVECTON HEATING 0F GLASS SURFACE MODULE PBESU RE PRDE ILE I V U/ AVERAGE SUFPORI PRESSQgE -ATMQSP ERIC RE$ y; 5

INVENTO J/iMES' C FAEJZE'Y BYGIOKGE' 1 5267619751? y 25, 1957 J. c. FREDLEY ETAL 3,332,761

METHOD OF-ANNEALIING SHEETS OF GLASS ON A DECREASING TEMPERATURE GAS SUPPORT Original Filed Nov. 9, 1962 17 Sheets-Sheet 14 58 RELATIVE movamcm OF cmss WITH RESPECT Q TO MODULES 2 UNIFORM SUPPORT PRESSURE-UNIFORM GAS TEMPERATURE FOR CONVECTION COOLING O GLASS SURFACE AT HlGH COOLlNG RATES yonuu: PkEsSuu PROFILE U I E UZ VEEQLEEuToRT Paassgke ATMOSPHERIC HEAT TRANSFER RATE PROFILE AVERAGE H T T SFER RATE INVENTORS W JAMES c. menus) -14 e-50m: E. sum/rm J. C. FREDLEY ETAL July 25, 1967 METHOD OF ANNEALING SHEETS OF GLASS ON A DECREASING TEMPERATURE GAS SUPPORT Original Filed Nov. 9, 1962 17 Sheets-Sheet 15 mN UE July 25, 1967 J. c. FRE EY ETAL 3,332,761

METHOD OF ANNEALING SHEE OF GLASS ON A DEGREASING EMPERATURE GAS SUPPORT I Original Filed Nov. 9. 19 1'7 Sheets-Sheet 16 FIQSI 60k6 f. SLE 167175 2 BY I arrat/vsr FIGZS y 25, 96 J. c. FREDLEY ETAL 3,332,761

METHOD OF ANNEALING SHEETS OF GLASS ON A DECREASING TEMPERATURE GAS SUPPORT Original Filed Nov. 9, 1962 17 Sheets-Sheet 17 FIG. 32

INVENTO JAMES C. FKEDLEY BYGEORGE E SLE/GWTEK A TTOZNA'Y United States Patent 3,332,761 METHOD OF ANNEALING SHEETS OF GLASS ON A DECREASING TEMPERATURE GAS SUPPORT James C. Fredley, Tarentum, and George E. Sleighter, Natrona Heights, Pa., assignors to Pittsburgh Plate Glass Company, Pittsburgh, Pa., a corporation of Pennsylvania Original application Nov. 9, 1962, Ser. No. 236,676, now Patent No. 3,223,501, dated Dec. 14, 1965. Divided and this application Oct. 22, 1965, Ser. No. 502,670

4 Claims. (CI. 6525) This application is a division of our copending application, Ser. No. 236,676, now Patent Number 3,223,501 entitled, Fabrication of Glass, filed Nov. 9, 1962, which in turn was a continuation-in-part of our copending application Ser. No. 236,103, filed Nov. 7, 1962, now abandoned, which application is, in turn, a continuation-in-part of our copending application Ser. No. 209,456, filed July 12, 1962, now abandoned, which application is, in turn,

a continuation-in-part of our copending application Ser.

No. 185,757, filed Apr. 6, 1962, now abandoned, which application is, in turn, a continuation-in-part of our copending application Ser. No. 172,235, filed February 9, 1962, which application is, in turn, a continuation-in-part of our copending application Ser. No. 139,901, filed Sept. 22, 1961, now abandoned. Reference is also made to our copending applications Ser. No. 139,902, filed Sept. 22, 1961, now abandoned; Ser. No. 140,135, filed Sept. 22, 1961, now abandoned; Ser. No. 175,938, filed Feb. 27, 1962, now abandoned; Ser. No. 176,050, filed Feb. 27, 1962, now abandoned; Ser. No. 178,997, filed Mar. 12, 1962, now abandoned; Ser. No. 185,448, filed Apr. 5, 1962, now abandoned; and Ser. No. 195,773, filed May 18, 1962, now abandoned, all of which applications are directed to related subject matter. All of the aforesaid applications referred to herein pursuant to 35 U.S.C. 120, are assigned to the assignee of this application.

This invention relates to the fabrication of glass and more particularly to heating of glass and to the transportation and/or support of hot glass sheets, especially glass at a deformation temperature. It is particularly concerned with such a process when combined with another operation such as annealing such sheets.

Sheets of glass may be fabricated through known manufacturing techniques of bending, tempering, annealing or coating and combinations of such techniques to form end products having characteristics and uses different from the original product. A common feature of these techniques is the heating of glass sheets to a temperature above that at which the major surfaces or the contour thereof will be changed by a deforming stress or contact with solids, hereinafter referred to as deformation temperature. For most plate and window glass this temperature is around 980 degrees Fahrenheit and above, but usually below a temperature at which the glass becomes molten.

Economic utillzation of fabricating equipment requires that the glass sheets undergoing treatment be conveyed while hot.

The necessity of conveying glass at high temperature has heretofore resulted in undesirable deformation or marring of the major surfaces of glass sheets being treated due to physical contact with supporting and conveying apparatus While the glass is at elevated temperatures. The instant invention overcomes this defect common to the know methods of heat treating glass sheets. In addition, this invention overcomes further disadvantages peculiar to some of the individual fabricating techniques.

Included in the instant invention are new and useful methods and apparatus for supporting and conveying hot glass. More specifically, methods and apparatus have been devised for supporting and conveying a sheet of glass on a film of gas while the glass is at or above deformation 3,332,761 Patented July 25, 1967 ice temperature. The film of gas uniformly supports the glass against undesired deformation and eliminates the necessity of contact of the major surfaces of the glass sheet with any solid object while the glass is subject to deformation or impairment. In this manner, the marring or distorting now associated with current flat glass fabricating processes has been eliminated.

In known processes of annealing, glass exhibiting undesirably high internal stresses is conveyed on rollers through a lehr where it is reheated substantially to its upper annealing range to allow stresses to relax and is then cooled in a controlled manner through the lower limit of its annealing range. The nonuniform support and the unavoidable slippage between the rollers and the softened glass results in distortion and surface marring of the sheets. In the manner herein disclosed, glass may be reheated and annealed without the attendant disadvantages of the known processes. This is accomplished by supporting and conveying the heated sheets of glass along a gas film bed which provides uniform support Without physical contact with the major surfaces of the glass.

In accordance with an embodiment of the invention, there is provided a plurality of evenly distributed zones of uniform nominal pressure on the lower side of the sheet adequate to support the sheet element undergoing treatment. Gas flows from a reservoir under higher pressure into such zones, being uniformly throttled between the reservoir and each zone to restrict the passage of gas between the two. Each zone constitutes a unit of support area with respect to the sheet to be supported and each has a reference surface at its margins common to the remainder. Within each zone, gas entering from the reservoir is diffused after throttling so as to avoid creation of localized jets normal to the reference surface and otherwise to equalize pressure and flow under normal conditions of operation. Provision is made for escape of the flow of gas emanating from each zone when covered by glass. In operation, the rate of flow of gas from the reservoir to each zone is maintained at such level that the average clearance between the reference surface and the glass sheet being supported is not less than 0.001 inch and not greater than 0.050 inch, normally not greater than 0.025 inch for glass having a thickness of /8 inch and above, and in any case never more than 50 to percent of the thickness of the supported glass.

More particularly, the invention contemplates such a gas support system in which glass is introduced onto the support area at a temperature below that at which its major surfaces will mar on physical contact with solid objects, the glass is heated above deformation temperature while supported primarily by gas and is then cooled until below deformation temperature before removal from the gas support. The system is particularly well adapted to heating flat glass in the form of sheets or the like in which the thickness ranges up to /2 to one inch and the length and breadth of the sheet generally are over 6 inches or one foot to as much as 5 or 10 feet or greater,-

optionally bending it by travel over a curved bed, then rapidly cooling the surfaces or quenching by utilizing relatively cold gas as the support medium, supplementing the cooling effect on the supported side by complementary flow of cold gas against the opposite side to equalize the heat transfer from the two major surfaces until the entire body is cool enough to prevent loss of temper or, in other words, redistribution of the stress differential set up between the surfaces and the interior of the glass body by differential rates of cooling.

Advantageously, heating of glass upon the gas support is accomplished by burning a controlled admixture of gas and air, introducing the hot products of combustion to the reservoir or plenum chamber which supplies the supporting zones, and supplementing the heat thus supplied 3 to the glass by radiant heat from an independently controlled source or sources which are generally disposed on the side of the glass opposite the supported side.

The attendant advantages of this invention and the various embodiments thereof will be readily appreciated as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings in which:

FIG. 1 is a perspective, partly schematic, view illustrating a system for conveying, heating and quenching sheet glass parts embodying several features of the present invention; FIG. lA is another partly schematic perspective on a larger scale illustrating particularly how sheet glass parts are driven by discs contacting an edge of the part while it is otherwise supported entirely by a gas film over the inclined bed of FIG. 1;

FIG. 2 is a detailed view partly in section and partly in elevation taken along the line 22 of'FIG. 1;

FIG. 3 is a partial plan view showing the arrangement of the preheat section with respect to gas film support heating section, the relative positions of the burners feeding combustion gases to the plenum chambers and the mechanism for conveying glass sheets by edge contact only;

FIG. 4 is a partial plan view which is in effect a continuation of FIG. 3 and shows the terminus of the gas film support heating section adjacent the quenching section, the latter being followed by the conveyor roll'run out sections;

FIG. 5 is a side elevation of the quenching system showing the relationship of the upper and lower heads;

FIG. 6 is an end elevation of the system of FIG. 5;

FIG. 7 is a sectional view-partly in elevation taken along the line 77 of FIG. 1;

FIG. 8 is a schematic view showing the arrangements for supplying air and cooling water to the quenching heads;

FIG. 9 is an approximately full scale sectional detail illustrating elements of quenching module design and pat terns of air flow during operation;

FIG. 10 is a sectional detail showing the arrangement for supplying air to those quenching modules in the row next adjacent to the heating section;

FIG. 11 is a partially detailed plan view of the first and second beds in the gas support heating section showing the relationship of the. individual modules in the geometry of the mosaic;

FIG. 12 is a sectional view taken along line 12-12 of FIG. 11, showing the relationship of the modules and exhaust ducts to the bed plate and plenum chamber;

7 FIG. 13 is an enlarged partial plan view of the lower quenching bed of FIG. 4;

FIG. 14 shows the arrangement used to vary the speed of the conveyor drive during the run out of parts from the heating to the quenching section;

FIG. 15 is a perspective view showing a gas film support bed, the generative surface of which progressively changes in contour from flat to a cylindrical shape in a cross section normal to the longitudinal axis of the bed;

FIG. 16 is an end elevation of the bed of FIG. 15 looking toward the part of maximum curvature;

FIG. 17 is a side elevation of the bed of FIG. 15 showing how the curve is developed along the path of travel of the glass;

FIG. 18 is an elevation of the burners, gas and air feeds and controls for one of the three plenum chambers of the gas support heating section;

FIG. 19 is a schematic view on enlarged scale of a section of the gas support bed showing diagrammatically the flow and exhaust of the support gases and presenting diagrammatic graphs in conjunction therewith;

FIG. 20 is a view similar to FIG. 19, presenting diagrammatic graphs and flows in conjunction with the quenching system;

FIG. 21 is a plan view approximately double scale illustrating a prototype support module unit;

FIG. 22 is a section taken along the line 22-22 of FIG. 21;

FIG. 23 is a plan view double scale of an improved support module unit, simpler of fabrication and in which the support area is subdivided by partitions;

FIG. 24- is a section taken along lines 2424 of FIG. 23;

FIG. 25 is a plan view approximately double scale of a typical quenching module unit;

FIG. 26 is a section taken along lines 2626 of FIG. 25;

FIG. 27 is a plan view of a quenching module unit having a step in the marginal walls to enhance turbulence of the quenching gases at the gas-glass interface;

FIG. 28 is a section taken along lines 2828 of FIG. 27;

FIG. 29 is a plan view approximately double scale illustrating a support module unit having a circular cross section in the plane of support;

FIG. 30 is a section taken along line 3030 of FIG. 29;

FIG. 31 is a partial plan view of a module bed of the module units shown in FIGS. 29 and 30;

FIG. 32 is a partial plan view of support modules arranged in rows with alternate longitudinal exhaust grooves; and

FIG. 33 is a section taken along line 33--33 of FIG. 31.

Referring to the drawings, FIG. 1 illustrates a system advantageously employed for heating fiat glass parts up to or above the deformation temperature, e.g.,'to a temperature at which the glass can be tempered, quenching such parts while hot and delivering the parts thus tempered onto a roll conveyor for removal. The component sections making up the complete system consist of a preheat section 1 wherein the glass is conveyed on rollers between radiant heaters to preheat the glass until brought to a suitable preheat temperature under the deformation temperature; a gas film support heating section 2, where the glass parts are transferred to, and supported on, a film of hot gas while being conveyed through a frictional drive contacting the edges only of such parts, supplemental heat being supplied by radiant heat sources above and below the glass until the glass reaches a temperature high enough for tempering purposes; a quenching section 3, where the glass is rapidly chilled while suspended between opposed flowing films of cool air, edge contact driving being continued through the section, and a delivery roll system 4 which receives the tempered glass parts from the quenching system and conveys them to their next destination.

Preheat section 1 includes an apron roll unit 5 for loading, the first few rolls being idle and the last driven. Next in order of the direction of travel of the workpiece are three identical enclosed preheat units 6 followed by three enclosed hot gas support heating units 7, the quenching section 3 and the delivery section 4.

For ease in fabrication, all units 5, 6, 7 and sections 3 and 4 are assembled Within rectilinear frameworks of support and mounted on casters 8 for convenience in assembly. Each unit and section is elevated from the casters 8 by jacks 9 into a position with the surfaces of all rolls and the gas support beds in a common plane tilted in a sidewise direction at an angle of five degrees with respect to the horizon as shown in FIGS. 1, 2, 6 and 7. The essentialframework consists of girders 11, stanchions 12, and beams 13 resting on support blocks 14.

THE PREHEAT SECTION Each unit 6 of the preheat section includes a radiant floor 16 and a radiant roof 17 built up from individual each unit 6 may be regulated as to temperature across the path of travel and parallel thereto. Each unit is provided with a thermocouple (not shown) to sense the temperature of the unit and the glass and to actuate the unit to the extent necessary to supply the required amount of heat. Conveyor rolls 20 are provided with guide collars 21 in alignment throughout the section 1 so as to position the glass properly for transfer to the gas support next following. Each roll is journaled in bearings 22 and is driven through gears 23 from a common shaft 24 energized by drive motor 25. Temperature sensing devices 26 (FIG. 7) placed at intervals along the path of travel of the workpiece aflord data from which to establish control.

GAS FILM SUPPORT HEATING SECTION To supply air under pressure to the hot gas support combustion system, each unit 7 (FIG. 3) employs a blower 50 feeding air under pressure through a butterfly control to a manifold 51. As best shown in FIG. 18, the individual burners 34 are supplied with air from the manifold through conduits 52, each provided with a valve 53 and an orifice at 54 of known size. Pressure drop across each orifice can be measured by manometers 55 affording means to determine individual flow rates. Pressure gauges 56 permit balancing of static pressures in the air flowing to the burners.

Gas from main 60 is introduced into each burner 34 via conduits 61 each individually valved as at 62 and provided with flow metering devices 63 connected to manometers 64.

Each burner 34 is of the so-called direct fired air heater type. Air from blower 50 is tapped into premixer 65 and there mingled with gas supplied through pipe 66 from the main 60 from whence the mixture flows to a manifold 67 connected to burner pilots 68 by inlets 69. Each pilot 68 is provided with a continuous type spark plug 70 for ignition and safety against blowouts in addition to which each burner contains a glow tube (not shown) which remains incandescent during operation to sustain flame within the burner. Gas to the pilot premixer is controlled through needle-valve 71 and shutoff valve 72. Sight ports 73 and 74 permit visual independent inspection of the pilot flame and main flame, respectively, in each burner. Diaphragm type safety devices 75 act to shut off all gas and air in the event of loss of either gas or air supply pressure.

The combustion of the products in the combustion chamber produces suflicient plenum pressure to supply the modules with heated gas of a uniform temperature and pressure. Adequate control of pressure and temperature are provided by correlating the rates of input of air and fuel to the burners. To supply enough gas to effect the desired support under normal conditions, an excess of air (usually 50 percent or more in excess) over that required for the combustion of the fuel gas is used. The supply of gas may be varied to change the heat input and the supply of air may be varied to change the pressure in the plenum.

The modules and plenum chamber are in most cases made of metal, such as iron, or like material having high heat conductivity and the modules themselves are in good heat conductive relationship to the plenum chamber, being connected thereto.

FIGS. -17 show a module bed 76 of a curved rath- .er than a flat contour for use in bending glass while it is 6 DELIVERY SECTION As shown in FIG. 1, the delivery roll section 4 consists of conveyor rolls 200 provided with guide collars 210 in alignment with discs 370 of the quench section to maintain the proper position of the glass during transfer therefrom. Each roll is journaled in bearings 220 and is driven through gears 230 from a common shaft 240 energized by drive motor 250.

MODULE DESIGN In accordance with an embodiment of this invention, highly developed and refined supporting .apparatus have been provided to prevent the distortion of glass at deformation temperature, an important achievement not accomplished by known conveying apparatus and processes, including known air film support devices. Specifically, it is important to have a very large proportion of the glass sheet or plate supported by a uniform force. This prohibits flowing the supporting air film across substantial areas of a supporting plate (i.e., between such a plate and the supported glass) because of the creation thereby of a progressive pressure drop along the path of flow and, hence, a nonuniform supporting force. Furthermore, air introduced from a plurality of points beneath the supported glass must be exhausted beneath the supported area rather than merelyby lateral flow to the glass edges to prevent a pressure build-up centrally of the supported sheet that will cause a doming effect upon the soft glass. The gas, having exhausted to points below the modules and adjacent the stems thereof, then flows principally to the sides of the bed through the exhaust channel 77 underneath the modules, some portion of the gas exhausting through :ducts 39. This channel 77 is disposed underneath the modules, the module stems 32 which extend therethrough being long enough to provide adequate height to this space.

Of course, if the support zones are small in comparison with the exhaust areas, the support pressure will not be substantially uniform. If the exhaust areas are large in magnitude, thinner sheets of glass overlying these areas will have a tendency to sag Conversely, if the support areas are too large and exhaust areas too small, doming of the glass tends to occur. Also, the pressure differential between the supporting pressure and the exhaust pressure must not be too great in order to avoid sagging.

Finally, it is important that the support be provided by a diffused and relatively small gas flow to provide substantially uniform pressure across the width of the support zone, thereby avoiding deformation, such as dimpling, from velocity pressure due to the direct impingement of localized jets of gas against the supported glass surface. The module embodiments illustrated in FIGS. 21 to 30, 32 and 33, when assembled to form a supporting bed and suitably supplied with gas from a plenum chamber in a manner to be described in more detail, provide the uniform support required to process glass at elevated temperatures substantially free from distortion in the manner herein disclosed.

As indicated by the embodiment depicted in FIGS. 21 and 22 and shown schematically in FIG. 19, each module 31 forms an open-topped chamber, being essentially closed on its other sides, the upper terminus of which defines a zone of substantially uniform pressure (a profile of which is diagrammatically shown in FIG. 19) beneath the overlying glass. The pressure is exerted by gas supplied to each module from the supporting plenum chamber by way of the hollow supporting stem 32. A nozzle 150, in threaded engagement with an opening 162 in the base of the module 31 and having a bore 163 connected with the bore 164 of module stem 32, provides a gas inlet to the module chamber and also functions to diffuse the gas by changing the direction of flow to a horizontal direction as the gas escapes and expands into the module chamber through a plurality of bores or 

1. A METHOD OF ANNEALING GLASS WHICH COMPRISES ESTABLISHING A GAS SUPPORT PATH SUPPLYING GAS AT PROGRESSIVELY LOWER TEMPERATURES TO SUCCESSIVE SECTIONS OF THE PATH THE GAS SUPPLIED TO THE FIRST OF SAID SECTIONS BEING NEAR THE UPPER LIMIT OF THE ANNEALING RANGE OF THE GLASS, THE GAS SUPPLIED TO THE LAST OF SAID SECTIONS BEING NOT OVER THE LOWER LIMIT OF SAID RANGE, AND MOVING THE GLASS AT AN INITIAL DEFORMATION TEMPERATURE OVER SAID PATH AND THEREBY COOLING THE GLASS OVER THE ANNEALING RANGE. 